shear pins vs. nylon screws

[mudmanc5]

pros and cons and who supplies them

 

[JoesRocketry]

I use #2-56 nylon screws. You can find them at McMaster carr (www.mcmaster.com). I can't really give you pros and cons on them, but If find them easy to use and they work rather well.


Also, some people use styrene rods and cut it flush with the airframe. The styrene rod method will create less drag and might be cheaper in the long run.

 

[eastvolt]

yep, i use 2-56 nylon screws from mcmaster too. work great!

 

[Rocketjunkie]

yep, i use 2-56 nylon screws from mcmaster too. work great!

I use the pan head ones, usually the 4-40 size. Easier on the thumb pushing them in

 

[Crazyrocket]

On all my rockets, I've used 1/16" styrene rod. Works very well. I typically use 3 or 4 on the nose cone. Never had any problems. The only reason I use the styrene rod is that I can get it cheaply at my local hobby shop. Try both and see what you like.

 

[bigone5500]

2-56 nylon screws....works every time...

 

[WillMarchant]

https://rocketmaterials.org/ has information about Nylon screws when used as shear pins.

I bought a handful of #2 and #4 Nylon screws from https://www.missileworks.com/

I'd order from https://www.mcmaster.com/ but whenever I do I end up adding a bunch of *other* stuff to my order so https://www.missileworks.com/ ends up being cheaper. :surprised:

 

[bigone5500]

Be sure that if you use nylon screws you soak the hole in CA and use a tap to create the threads. The CA will aid in making better threads and also strengthen the hole. I bought 100 2-56 screws, a #51 drill bit, and a 2-56 taper tap for less than $15 from Grainger.

 

[JimJarvis50]

On my glassed phenolic and composite airframes, I use the #2 nylon screws. I drill a 5/64 hole and then just tap in the screws with a hammer. It's about as easy as can be.

On my last order from McMaster, I ordered several types of #2 screws (different colors). Part of the order was a different size than the normal screws I've been getting for several years now. They were a little undersized and didn't fit my 5/64 holes. Just a heads-up.

 

[Tom Rouse]

WOW, everyones shear pin experiences varies drastically.
I use .080" round styrene rods. Perfect.

Greg Fannin, TRA's guru of class, uses this also. Greg's rockets are the best in the whole TRA organization. Bar none. Thius guy is good.
he once used three shear pins and ground tested his 4 inch min dia rocket with a 16 gram CO2. It shot it across the entire yard!

Well, the extra shear pin kept the parts together longer.
Remember in Hiogh School F=M X A?
If the mass stays the same, the acceleration will go berserk when the force is increased!. So, an extra shear pin will get the parts screaming apart. Unless you put too many on and it keeps ot from seperating.
Test, test. Keep the vent holes small.
Or, you can always ask Greg.

The styrene rods are made by "Plastruct", City of Industry, Ca.
The nylon pins, the other "screw" pins all work. But, you might want to be sure your application works for the rocket you've built.
If you have a sloppy coupler/airframe fit, well, you're hosed no matter what.
Question-
"How many of you test your shear pojuns manually?". Sheez. A given.
POut the shear pins in and pull your rockets parts arart to "feel" the tension and separation forces. You will be amazed how it works.
A tight fit of the coupler and body tube is important so it SHEARS the pins and does not 'tear" them.
Hand test the pins, then ejection test charge them.
T

[edited by moderator]

 

[JimJarvis50]

WOW, everyones shear pin experiences varies drastically.QUOTE]

I have always been very picky about my shear pins and other things rocket. That's just me. I once helped another very experienced flier load up a rocket. He was missing a shear pin, and he reached down and picked up a twig. There's just more than one way to do it.

 

[jcsalem]

I've tried both #2 nylon screws (from McMaster-Carr) and 1/16" styrene rod from the local hobby shop. The 1/16" styrene shears MUCH more easily than the #2 screws. Too easily -- after having a mistaken apogee deployment I switched to the #2 screws exclusively.

I've had a couple cases where there wasn't enough pressure to shear the #2 screws. But since I use the shear pins just for the main parachute on DD rockets and since I fly on relatively small fields, I prefer no main over a main deployed at apogee.

Jim

 

[RimfireJim]

I've tried both #2 nylon screws (from McMaster-Carr) and 1/16" styrene rod from the local hobby shop. The 1/16" styrene shears MUCH more easily than the #2 screws.

That makes sense from a stress/strength calculation standpoint. A 1/16" diameter rod has about 80% of the shear stress area of a #2 screw, and Nylon 6/6 is roughly 2X stronger than styrene, so when you combine those two facts, the styrene rod should break at about 40% of the load at which the screw will break.

A 3/32" diameter styrene rod should come close to having the same shear strength as a #2 Nylon screw. But, to quote from Shigley, Joseph E. "Mechanical Engineering Design", 4th ed., p. 368, "Remember, a good engineer never went wrong confirming his or her analysis with lots of good laboratory tests."

 

[pbahorich]

I like the styrene rod concept since it provides a more repeatable shear stress due to the constant cross sectional area. How do you keep them from falling out of the holes (including high Mach number flights)?

 

[manixFan]

The advantage of the screws is the shear strength is often available from the maker. Much harder to find with styrene rod. I use both, rod on smaller rockets and screws on heavier ones. The ejection charge calculator I use will tell me how many 2-56 screws the charge will shear, which obviously is very useful.


Tony

 

[heada]

I've started using M2 screws. Nylon slotted screws for sheer pins and 304 stainless phillips for retaining screws. Both are screwed into 316 stainless tubing that has been tapped for M2 threads. On the points I want to sheer, I also use 316 stainless 0.015" shim stock on the mating surface. This acts as a scisoring action to ensure the nylon screw is sheered off clean and keeps the holes from elongating. To remove the screw body, I use a jeweler's screw driver that has been heated, insert it into the end of the screw and it melts a new slot that I can use to turn the screw body free. Everything is available from McMaster-Carr for fairly cheap and in bulk (100 screws is the minimum order)

 

[pbahorich]

I also want to keep the surface flush for aerodynamic reasons. Has anyone used countersunk nylon screws, or drilled the outer tube large enough to hold the head of a pan head screw?

 

[heada]

I also want to keep the surface flush for aerodynamic reasons. Has anyone used countersunk nylon screws, or drilled the outer tube large enough to hold the head of a pan head screw?

That's exactly what I do with the M2 screws. Countersunk flush with the outer body tube.

https://www.mcmaster.com/92929A238-95258A108

 

[Rocketjunkie]

I also want to keep the surface flush for aerodynamic reasons. Has anyone used countersunk nylon screws, or drilled the outer tube large enough to hold the head of a pan head screw?

You can also use set screws. However, I use 2-56 or 4-40 pan head screws and a press fit (#2 is 5/64" for a tight fit, #4 is 7/64"). If I need aerodynamic performance, I cut the head off. Removal is as simple as poking the remnants through.

 

[pbahorich]

Press fit and cutting the heads off is extremely simple and I love that.
Have you used this method at high Mach numbers?
How many flight cycles do you get out of this before the fiberglass tube gets worn away too much for the press fit to work?

 

[FredA]

Press fit and cutting the heads off is extremely simple and I love that.
Have you used this method at high Mach numbers?
How many flight cycles do you get out of this before the fiberglass tube gets worn away too much for the press fit to work?

I do this too.
Flights to M3+
Usually just rotate the NC or EBay to a new position and drill a new set of inner holes for each flight. This starts to look a bit like Swiss cheese after 20-30 flights, but who cares.

Steel set screws for aerodynamically smooth hard connections - otherwise just pan heads -- both just screwed into tapped FG/CF holes without extra hardware.

 

[Rocketjunkie]

Press fit and cutting the heads off is extremely simple and I love that.
Have you used this method at high Mach numbers?
How many flight cycles do you get out of this before the fiberglass tube gets worn away too much for the press fit to work?

With fiberglass airframes you won't have any problems. Cardboard, just harden the holes with CA, drill new ones as needed. FredA also responded. To reuse the same holes, just poke out the remnants and install new screws.